Cardiac pacing systems include a cardiac pacer connected to a lead which conducts stimuli from the pacer to an electrode implanted either on the surface of or within the cardiac chambers. Certain cardiac pacers, including those designated VOO pacers, emit stimuli at a predetermined rate, in pulses per minute. However, there are certain other models, including those designated VVI pacers, in which the output pulse is suppressed in the presence of naturally occurring cardiac electrical activity. Such models detect cardiac electrical activity via the lead during a portion of the interval termed the alert period between output pulses. When such activity is detected during the appropriate portion of the pulse to pulse interval, a new timing cycle is initiated with time zero of the new interval being the time of detection of the electrical activity. Such electrical activity detected in, e.g., the ventrical, may be that which normally occurs from the atrium to the ventricle or may be due to a premature ventricular or atrial contraction dependent upon the lead placement and pacer type.
Such cardiac pacers which detect electrical activity during the alert period must have a sensitivity threshold for enabling such detection. The sensitivity of a cardiac pacer is generally considered to be that level, in millivolts, which the electrical activity resulting from the depolarization of the cardiac muscle must exceed within a given time period for the cardiac depolarization to be detected by the sensing amplifier of the cardiac pacer. For example, if the sensitivity of a cardiac pacer has been set at one millivolt, then the cardiac electrical depolarization must obtain the one millivolt level within, for example, a 30 milliseconds time period to be detected by the cardiac pacer, thereby inhibiting the output stimulus and resetting the timing cycle of the cardiac pacer.
The sensitivity of these "sensing" cardiac pacers may be preset during manufacture. In other models of sensing cardiac pacers, the sensitivity may be externally programmed to various levels. Such externally programmed changes in sensitivity are accomplished by the use of an external device and usually require a visit to the physician's office. Therefore, there is no automatic change in sensitivity levels if spontaneous myocardial electrical activity ceases to be detected by the implanted cardiac pacer.
The importance of proper sensitivity adjustment cannot be overemphasized. If the sense amplifier of the pacer is not sensitive enough, proper sensing of cardiac activity will not occur. On the other hand, if the sense amplifier is too sensitive, it may sense noise and/or myopotentials. Although a physician may be able to readjust for proper sensitivity of the pacer, many patients are unable to see their physicians often. Thus although the sense amplifier of a pacer is presumably programmed correctly at a particular time, the cardiac coupling parameters may change at a subsequent period of time (as in fibrotic tissue growth) and the sensitivity of the sense amplifier may consequently become either too low or too high.
The present invention obviates the need for a number of tuned amplifiers on a signal channel, and also obviates the need for utilizing a tuned amplifier in which the gain must be varied. In a system requiring the varying of the gain of a tuned amplifier, in addition to requiring many components there may be band pass detuning problems. Further, systems requiring multiple amplifiers or gain varying circuitry may suffer from lack of resolution because circuitry tends to become unwieldy for six or more sensitivity levels.
Therefore, it is an object of the present invention to provide a cardiac pacer in which sensitivity is automatically adjusted as required to optimally sense spontaneous cardiac electrical activity without requiring unwieldy circuitry.
Another object of the present invention is to provide an automatic sensitivity adjusting cardiac pacer in which a single fixed gain, tuned amplifier may be utilized. This may result in comparatively fewer components and the circuit of the present invention enables a resolution of greater than 250 sensitivity levels when an eight bit system is used. Instead of varying sense amplifier gain, the DC reference voltage at the inputs to a pair of sense detection comparators may be programmed and these DC levels may be under the direct software control of an N-bit digital to analog converter.
Other objects and advantages of the present invention may become apparent as the description proceeds.